Controlled and Selective Photo-oxidation of Amyloid-β Fibrils by Oligomeric p-Phenylene Ethynylenes

Abstract

Photodynamic therapy (PDT) has been explored as a therapeutic strategy to clear toxic amyloid aggregates involved in neurodegenerative disorders such as Alzheimer's disease. A major limitation of PDT is off-target oxidation, which can be lethal for the surrounding cells. We have shown that a novel class of oligo-pphenylene ethynylenes (OPEs) exhibit selective binding and fluorescence turn-on in the presence of prefibrillar and fibrillar aggregates of disease-relevant proteins such as amyloid-β (Aβ) and αsynuclein. Concomitant with fluorescence turn-on, OPE also photosensitizes singlet oxygen under illumination through the generation of a triplet state, pointing to the potential application of OPEs as photosensitizers in PDT. Herein, we investigated the photosensitizing activity of an anionic OPE for the photo-oxidation of Aβ fibrils and compared its efficacy to the well-known but nonselective photosensitizer methylene blue (MB). Our results show that, while MB photo-oxidized both monomeric and fibrillar conformers of Aβ40, OPE oxidized only Aβ40 fibrils, targeting two histidine residues on the fibril surface and a methionine residue located in the fibril core. Oxidized fibrils were shorter and more dispersed but retained the characteristic β-sheet rich fibrillar structure and the ability to seed further fibril growth. Importantly, the oxidized fibrils displayed low toxicity. We have thus discovered a class of novel theranostics for the simultaneous detection and oxidization of amyloid aggregates. Importantly, the selectivity of OPE's photosensitizing activity overcomes the limitation of off-target oxidation of traditional photosensitizers and represents an advancement of PDT as a viable strategy to treat neurodegenerative disorders.

Partial Destabilization of Amyloid-β Protofibril by Methionine Photo-oxidation: A Molecular Dynamic Simulation Study

“…In contrast, photo-oxidation of A fibrils that results in Met35 oxidation has been observed to break down fibrils into smaller fibrils that retained their cross--sheet structure. 43 Our simulation results are thus consistent with the experimental finding that the intra-and inter-peptide -sheet structures that form the backbone of the fibrils were not completed disrupted by Met35 photooxidation.…”

“…We have shown in a recent in vitro study that a fibril selective photosensitizer caused clumps of A40 fibrils to dissociate and fragment into smaller fibrils with light irradiation. 43 Moreover, the oxidized fibrils retained a significant amount of -sheet structures of the native fibrils and the ability to seed the aggregation of A monomers. This partial fibril destabilization may be advantageous since more complete degradation of amyloid fibrils can potentially result in oligomers that are more toxic compared to monomeric or fibrillar Aβ conformers.…”

“…40,41 Recently, several aggregate-selective photosensitizers have been developed and tested, including those based on fibril-binding dyes thioflavin-T and curcumin 37,38,42 and amyloid aggregate selective p-phenylene ethynylene-based florescence sensors. [43][44][45] These new aggregate-selective photosensitizers can potentially overcome off-target oxidation and minimize side effects in future clinical applications.…”

“…Many studies have documented the morphological changes to the fibrillar structure of A upon photo-oxidation, including fibril fragmentation, rupture, and disintegration. 36,37,43 Photooxidation sites have also been identified. However, molecular level details of the conformational changes that leads to fibril destabilization have not been elucidated.…”

“…54 For Aβ1-40 fibrils, we and others have shown that photosensitization results in the oxidation of two histidine residues (His13 and His14) and Met35. 18,43,46 His13 and His14 are located on the surface of A fibrils whereas Met35 is located in the hydrophobic core region of the fibrils.…”